Paizo Core

Though most think of the Plane of Air as a
vast expanse of clear sky, that plane also holds the power of the storm,
including monstrous hurricanes larger than entire worlds and
thunderstorms whose peals and claps can shatter stone. Whether lightning elementals are the by-product of common air elementals
spending too much time near these electrically charged storms or are
merely calved off like forgotten flurries, they are aggressive and
almost suicidal in their willingness to leap into battle. They
particularly enjoy attacking creatures that are made of metal and creatures that wear metal armor or wield metal weapons.

Many lightning elementals have a roughly
humanoid shape, but some prefer animalistic shapes (particularly birds
and dragons), and a few appear to be nothing more than large disembodied
heads made of dark clouds with flickering lightning tongues.
Regardless of the shape a lightning elemental takes, the air around the
creature hums and throbs with the promise of electrocution and the
tangy stink of ozone.